lua cache bindings: error out if cache isn't open yet

The catch is that during configuration file processing,
no cache is open (yet), as kresd can't know if the config
does open it in some later part (with non-default path or size).
Now we just throw an error.  Exceptions:
 - cache.open() and cache.backends(), of course :-)
 - cache.ns_tout() - not required, it's not really inside cache
 - cache.close() - it sounds reasonable to allow "closing a closed cache"

This immediately caught a typo in cache metatable.